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                <h1 id="二、TypeScript-基础语法入门"><a href="#二、TypeScript-基础语法入门" class="headerlink" title="二、TypeScript 基础语法入门"></a>二、TypeScript 基础语法入门</h1><h2 id="2-9类型注解和类型推断"><a href="#2-9类型注解和类型推断" class="headerlink" title="2.9类型注解和类型推断"></a>2.9类型注解和类型推断</h2><h3 id="2-9-1-类型注解"><a href="#2-9-1-类型注解" class="headerlink" title="2.9.1 类型注解"></a>2.9.1 类型注解</h3><p>什么是类型注解，类型注解就是人工的告诉TS，变量或者对象的明确属性类型</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">const</span> userName<span class="token operator">:</span><span class="token builtin">string</span><span class="token operator">=</span><span class="token string">'23'</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<h3 id="2-9-2-类型推断"><a href="#2-9-2-类型推断" class="headerlink" title="2.9.2 类型推断"></a>2.9.2 类型推断</h3><p>如果类型推断能够自动推断出来的类型就没有手写类型注解</p>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">const</span> userNick<span class="token operator">=</span><span class="token string">'Dell'</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><p>假如类型推断不符合要求，那么我们将对代码进行类型注解。</p>
<h2 id="2-10-类型收窄"><a href="#2-10-类型收窄" class="headerlink" title="2.10 类型收窄"></a>2.10 类型收窄</h2><p>将类型推导为更精准的类型的过程，我们称之为收窄</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">padLeft</span><span class="token punctuation">(</span>padding<span class="token operator">:</span><span class="token builtin">number</span><span class="token operator">|</span><span class="token builtin">string</span><span class="token punctuation">,</span>input<span class="token operator">:</span><span class="token builtin">string</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> padding<span class="token operator">===</span><span class="token string">"number"</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name"><span class="token builtin">Array</span></span><span class="token punctuation">(</span>padding<span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">" "</span><span class="token punctuation">)</span><span class="token operator">+</span>input<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> padding <span class="token operator">+</span> input<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="2-10-1-typeof类型保护"><a href="#2-10-1-typeof类型保护" class="headerlink" title="2.10.1 typeof类型保护"></a>2.10.1 typeof类型保护</h3><p>在ts中，检查typeof返回值就是一种类型保护。ts知道typeof不同值的结果，它也能识别JavaScript中一些怪异的地方</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">padLeft</span><span class="token punctuation">(</span>padding<span class="token operator">:</span> <span class="token builtin">number</span> <span class="token operator">|</span> <span class="token builtin">string</span><span class="token punctuation">,</span> input<span class="token operator">:</span> <span class="token builtin">string</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> padding <span class="token operator">===</span> <span class="token string">"number"</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name"><span class="token builtin">Array</span></span><span class="token punctuation">(</span>padding <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">" "</span><span class="token punctuation">)</span> <span class="token operator">+</span> input
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> padding <span class="token operator">+</span> input<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="2-10-2-真值收窄"><a href="#2-10-2-真值收窄" class="headerlink" title="2.10.2 真值收窄"></a>2.10.2 真值收窄</h3><p>if语句不需要条件的结果总是布尔类型，这是因为javaScript会做隐式类型转换，像 <code>0</code> 、<code>NaN</code>、<code>""</code>、<code>0n</code>、<code>null</code> <code>undefined</code> 这些值都会被转为 <code>false</code>，其他的值则会被转为 <code>true</code>。</p>
<p>也可以使用 <code>Boolean</code> 函数强制转为 <code>boolean</code> 值，或者使用更加简短的<code>!!</code></p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">multiplyAll</span><span class="token punctuation">(</span>
    values<span class="token operator">:</span><span class="token builtin">number</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token operator">|</span><span class="token keyword">undefined</span><span class="token punctuation">,</span>
    factor<span class="token operator">:</span><span class="token builtin">number</span>
<span class="token punctuation">)</span><span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">|</span> <span class="token keyword">undefined</span><span class="token punctuation">{</span>
   <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token operator">!</span>values<span class="token punctuation">)</span><span class="token punctuation">{</span>
     <span class="token keyword">return</span> values
   <span class="token punctuation">}</span>
   <span class="token keyword">return</span> values<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>x<span class="token operator">=&gt;</span>x<span class="token operator">*</span>factor<span class="token punctuation">)</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="2-10-3-等值收窄"><a href="#2-10-3-等值收窄" class="headerlink" title="2.10.3 等值收窄"></a>2.10.3 等值收窄</h3><p>typescript 也会使用 <code>switch</code> 语句和等值检查比如 <code>==</code> <code>!==</code> <code>==</code> <code>!=</code> 去收窄类型</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">example</span><span class="token punctuation">(</span>x<span class="token operator">:</span><span class="token builtin">string</span><span class="token operator">|</span><span class="token builtin">number</span><span class="token punctuation">,</span>y<span class="token operator">:</span><span class="token builtin">string</span><span class="token operator">|</span><span class="token builtin">boolean</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
   <span class="token keyword">if</span><span class="token punctuation">(</span>x<span class="token operator">===</span>y<span class="token punctuation">)</span><span class="token punctuation">{</span>
     <span class="token comment">// 当两值相等时，两值均为string类型</span>
     x<span class="token punctuation">.</span><span class="token function">toLowerCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
     y<span class="token punctuation">.</span><span class="token function">toLowerCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
    <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>y<span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>判断具体的字面量值也能让 TypeScript 正确的判断类型。</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">printAll</span><span class="token punctuation">(</span>strs<span class="token operator">:</span><span class="token builtin">string</span><span class="token operator">|</span> <span class="token builtin">string</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">|</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
  <span class="token keyword">if</span><span class="token punctuation">(</span>strs<span class="token operator">!==</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
     <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> strs <span class="token operator">===</span><span class="token string">"object"</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">const</span> s <span class="token keyword">of</span> strs<span class="token punctuation">)</span><span class="token punctuation">{</span>
          <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
     <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token keyword">else</span> <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> strs<span class="token operator">===</span><span class="token string">"string"</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
     <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>strs<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>此时上述的例子中，就可以正常处理<code>strs</code>的值为空字符串的情况。</p>
<p>JavaScript 的宽松相等操作符如 <code>==</code> 和 <code>!=</code> 也可以正确的收窄。</p>
<p>不过在 JavaScript 中，通过 <code>== null</code> 这种方式并不能准确的判断出这个值就是 <code>null</code>，它也有可能是 <code>undefined</code> 。对 <code>== undefined</code> 也是一样。</p>
<p>所以我觉得在以后的代码直接用<code>===</code>严格进行条件判断就ok了。</p>
<h3 id="2-10-4-in操作符收窄"><a href="#2-10-4-in操作符收窄" class="headerlink" title="2.10.4 in操作符收窄"></a>2.10.4 in操作符收窄</h3><p>JavaScript 中有一个 <code>in</code> 操作符可以判断一个对象是否有对应的属性名。TypeScript 也可以通过这个收窄类型。</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">type</span> <span class="token class-name">Fish</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token function-variable function">swim</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token keyword">void</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">type</span> <span class="token class-name">Bird</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token function-variable function">fly</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token keyword">void</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
 
<span class="token keyword">function</span> <span class="token function">move</span><span class="token punctuation">(</span>animal<span class="token operator">:</span> Fish <span class="token operator">|</span> Bird<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token string">"swim"</span> <span class="token keyword">in</span> animal<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> animal<span class="token punctuation">.</span><span class="token function">swim</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// (parameter) animal: Fish</span>
  <span class="token punctuation">}</span>
 
  <span class="token keyword">return</span> animal<span class="token punctuation">.</span><span class="token function">fly</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// (parameter) animal: Bird</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="2-10-5-instanceof-收窄"><a href="#2-10-5-instanceof-收窄" class="headerlink" title="2.10.5 instanceof 收窄"></a>2.10.5 instanceof 收窄</h3><p><code>instanceof</code> 也是一种类型保护，TypeScript 也可以通过识别 <code>instanceof</code> 正确的类型收窄</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">logValue</span><span class="token punctuation">(</span>x<span class="token operator">:</span>Date<span class="token operator">|</span><span class="token builtin">string</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
   <span class="token keyword">if</span><span class="token punctuation">(</span>x <span class="token keyword">instanceof</span> <span class="token class-name">Date</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
     <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">.</span><span class="token function">toUTCString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
    <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">.</span><span class="token function">toUpperCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="2-10-6-赋值语句（Assignments）"><a href="#2-10-6-赋值语句（Assignments）" class="headerlink" title="2.10.6 赋值语句（Assignments）"></a>2.10.6 赋值语句（Assignments）</h3><p>TypeScript 可以根据赋值语句的右值，正确的收窄左值。</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token comment">// 这里将x推导为string | numner</span>
<span class="token keyword">let</span> x<span class="token operator">=</span>Math<span class="token punctuation">.</span><span class="token function">random</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">&lt;</span><span class="token number">0.5</span><span class="token operator">?</span><span class="token number">10</span><span class="token operator">:</span><span class="token string">'叶春锁'</span><span class="token punctuation">;</span>

<span class="token comment">// 所以下面我们不管将其赋值为string类型数据还是number类型数据，这都是可行的</span>

x<span class="token operator">=</span><span class="token number">1</span><span class="token punctuation">;</span>

<span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>

x<span class="token operator">=</span><span class="token string">"哈哈哈"</span><span class="token punctuation">;</span>

<span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="2-10-7-控制流分析（Control-flow-analysis）"><a href="#2-10-7-控制流分析（Control-flow-analysis）" class="headerlink" title="2.10.7 控制流分析（Control flow analysis）"></a>2.10.7 控制流分析（Control flow analysis）</h3><pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">padLeft</span><span class="token punctuation">(</span>padding<span class="token operator">:</span> <span class="token builtin">number</span> <span class="token operator">|</span> <span class="token builtin">string</span><span class="token punctuation">,</span> input<span class="token operator">:</span> <span class="token builtin">string</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> padding <span class="token operator">===</span> <span class="token string">"number"</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name"><span class="token builtin">Array</span></span><span class="token punctuation">(</span>padding <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">" "</span><span class="token punctuation">)</span> <span class="token operator">+</span> input<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> padding <span class="token operator">+</span> input<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>在第一个 <code>if</code> 语句里，因为有 <code>return</code> 语句，TypeScript 就能通过代码分析，判断出在剩余的部分 <code>return padding + input</code> ，如果 padding 是 <code>number</code> 类型，是无法达到 (<strong>unreachable</strong>) 这里的，所以在剩余的部分，就会将 <code>number</code>类型从 <code>number | string</code> 类型中删除掉。</p>
<p><strong>这种基于可达性(reachability) 的代码分析就叫做控制流分析(control flow analysis)。</strong></p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">example</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">let</span> x<span class="token operator">:</span><span class="token builtin">string</span><span class="token operator">|</span><span class="token builtin">number</span><span class="token operator">|</span><span class="token builtin">boolean</span><span class="token punctuation">;</span>
    x<span class="token operator">=</span>Math<span class="token punctuation">.</span><span class="token function">random</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">&lt;</span><span class="token number">0.5</span><span class="token punctuation">;</span>
    <span class="token comment">// 这里x推导为boolean</span>
    <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span>Math<span class="token punctuation">.</span><span class="token function">random</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">&lt;</span><span class="token number">0.5</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
      x<span class="token operator">=</span><span class="token string">"hello"</span><span class="token punctuation">;</span>
      <span class="token comment">// 这里x是string类型</span>
      <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
        x<span class="token operator">=</span><span class="token number">100</span><span class="token punctuation">;</span>
        <span class="token comment">// 这里x是number</span>
        <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token comment">// 所以返回值将是string|number 类型。</span>
    <span class="token keyword">return</span> x<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="2-10-8-类型谓词-type-predicates"><a href="#2-10-8-类型谓词-type-predicates" class="headerlink" title="2.10.8 类型谓词(type predicates)"></a>2.10.8 <strong>类型谓词</strong>(type predicates)</h3><p>所谓 <code>predicate</code> 就是一个返回 <code>boolean</code> 值的函数。</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">type</span> <span class="token class-name">Fish</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token function-variable function">swim</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token keyword">void</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">type</span> <span class="token class-name">Bird</span> <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token function-variable function">fly</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token keyword">void</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// pet is Fish 就是类型谓词</span>
<span class="token comment">// 格式为parameterName is Type</span>
<span class="token comment">// 其中parameterName 必须是当前函数的参数名</span>
<span class="token keyword">function</span> <span class="token function">isFish</span><span class="token punctuation">(</span>pet<span class="token operator">:</span> Fish <span class="token operator">|</span> Bird<span class="token punctuation">)</span><span class="token operator">:</span> pet <span class="token keyword">is</span> Fish <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token string">'swim'</span> <span class="token keyword">in</span> pet<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="2-10-9-类型谓词-type-predicates"><a href="#2-10-9-类型谓词-type-predicates" class="headerlink" title="2.10.9 类型谓词(type predicates)"></a>2.10.9 <strong>类型谓词</strong>(type predicates)</h3><p>当联合类型中的每个类型，都包含了一个共同的<strong>字面量类型的属性</strong>，TypeScript 就会认为这是一个<strong>可辨别联合（discriminated union）</strong>，然后可以将具体成员的类型进行收窄。</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">interface</span> <span class="token class-name">Circle</span> <span class="token punctuation">{</span>
  kind<span class="token operator">:</span> <span class="token string">"circle"</span><span class="token punctuation">;</span>
  radius<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
 
<span class="token keyword">interface</span> <span class="token class-name">Square</span> <span class="token punctuation">{</span>
  kind<span class="token operator">:</span> <span class="token string">"square"</span><span class="token punctuation">;</span>
  sideLength<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
 
<span class="token keyword">type</span> <span class="token class-name">Shape</span> <span class="token operator">=</span> Circle <span class="token operator">|</span> Square<span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="2-10-9-可辨别联合（Discriminated-unions）"><a href="#2-10-9-可辨别联合（Discriminated-unions）" class="headerlink" title="2.10.9 可辨别联合（Discriminated unions）"></a>2.10.9 可辨别联合（Discriminated unions）</h3><p>当联合类型中的每个类型，都包含了一个共同的<strong>字面量类型的属性</strong>，TypeScript 就会认为这是一个<strong>可辨别联合（discriminated union）</strong>，然后可以将具体成员的类型进行收窄。</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">interface</span> <span class="token class-name">Circle</span> <span class="token punctuation">{</span>
  kind<span class="token operator">:</span> <span class="token string">"circle"</span><span class="token punctuation">;</span>
  radius<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
 
<span class="token keyword">interface</span> <span class="token class-name">Square</span> <span class="token punctuation">{</span>
  kind<span class="token operator">:</span> <span class="token string">"square"</span><span class="token punctuation">;</span>
  sideLength<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
 
<span class="token keyword">type</span> <span class="token class-name">Shape</span> <span class="token operator">=</span> Circle <span class="token operator">|</span> Square<span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>在这个例子中，<code>kind</code> 就是这个公共的属性（作为 Shape 的<strong>可辨别(discriminant)</strong> 属性 ）</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">interface</span> <span class="token class-name">Circle</span> <span class="token punctuation">{</span>
  kind<span class="token operator">:</span> <span class="token string">"circle"</span><span class="token punctuation">;</span>
  radius<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">interface</span> <span class="token class-name">Square</span> <span class="token punctuation">{</span>
  kind<span class="token operator">:</span> <span class="token string">"square"</span><span class="token punctuation">;</span>
  sideLength<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">type</span> <span class="token class-name">Shape</span> <span class="token operator">=</span> Circle <span class="token operator">|</span> Square<span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">getArea</span><span class="token punctuation">(</span>shape<span class="token operator">:</span> Shape<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>shape<span class="token punctuation">.</span>kind <span class="token operator">===</span> <span class="token string">"circle"</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token constant">PI</span> <span class="token operator">*</span> shape<span class="token punctuation">.</span>radius <span class="token operator">**</span> <span class="token number">2</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> shape<span class="token punctuation">.</span>sideLength <span class="token operator">**</span> <span class="token number">2</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="2-10-10-never-类型"><a href="#2-10-10-never-类型" class="headerlink" title="2.10.10 never 类型"></a>2.10.10 never 类型</h3><p>当进行收窄的时候，如果你把所有可能的类型都穷尽了，TypeScript 会使用一个 <code>never</code> 类型来表示一个不可能存在的状态。</p>
<h4 id="穷尽检查（Exhaustiveness-checking）"><a href="#穷尽检查（Exhaustiveness-checking）" class="headerlink" title="穷尽检查（Exhaustiveness checking）"></a>穷尽检查（Exhaustiveness checking）</h4><p>除了 <code>never</code> 自身，没有类型可以赋值给 <code>never</code>。这就意味着你可以在 <code>switch</code> 语句中使用 <code>never</code> 来做一个穷尽检查。</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">interface</span> <span class="token class-name">Circle</span> <span class="token punctuation">{</span>
  kind<span class="token operator">:</span> <span class="token string">"circle"</span><span class="token punctuation">;</span>
  radius<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">interface</span> <span class="token class-name">Square</span> <span class="token punctuation">{</span>
  kind<span class="token operator">:</span> <span class="token string">"square"</span><span class="token punctuation">;</span>
  sideLength<span class="token operator">:</span> <span class="token builtin">number</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">type</span> <span class="token class-name">Shape</span> <span class="token operator">=</span> Circle <span class="token operator">|</span> Square<span class="token punctuation">;</span>
 
<span class="token keyword">function</span> <span class="token function">getArea</span><span class="token punctuation">(</span>shape<span class="token operator">:</span> Shape<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">switch</span> <span class="token punctuation">(</span>shape<span class="token punctuation">.</span>kind<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">case</span> <span class="token string">"circle"</span><span class="token operator">:</span>
      <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token constant">PI</span> <span class="token operator">*</span> shape<span class="token punctuation">.</span>radius <span class="token operator">**</span> <span class="token number">2</span><span class="token punctuation">;</span>
    <span class="token keyword">case</span> <span class="token string">"square"</span><span class="token operator">:</span>
      <span class="token keyword">return</span> shape<span class="token punctuation">.</span>sideLength <span class="token operator">**</span> <span class="token number">2</span><span class="token punctuation">;</span>
      
    <span class="token comment">// 只有当Shape的所有类型都被处理完毕</span>
    <span class="token comment">// default分支中的shape的类型才会是never</span>
    <span class="token comment">// 当我们给 Shape 类型添加一个新成员，却没有做对应处理的时候，就会导致一个 TypeScript 错误	</span>
    <span class="token keyword">default</span><span class="token operator">:</span>
      <span class="token keyword">const</span> _exhaustiveCheck<span class="token operator">:</span> <span class="token builtin">never</span> <span class="token operator">=</span> shape<span class="token punctuation">;</span>
      <span class="token keyword">return</span> _exhaustiveCheck<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="2-11-对象相关知识"><a href="#2-11-对象相关知识" class="headerlink" title="2.11 对象相关知识"></a>2.11 对象相关知识</h2><p>对象类型解构的代码怎么写</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">getObjectValue</span><span class="token punctuation">(</span><span class="token punctuation">{</span>a<span class="token punctuation">,</span>b<span class="token punctuation">}</span><span class="token operator">:</span><span class="token punctuation">{</span>a<span class="token operator">:</span><span class="token builtin">string</span><span class="token punctuation">,</span>b<span class="token operator">:</span><span class="token builtin">string</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
  <span class="token keyword">return</span> a <span class="token operator">+</span> b<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token function">getObjectValue</span><span class="token punctuation">(</span><span class="token punctuation">{</span>a<span class="token operator">:</span><span class="token string">'1'</span><span class="token punctuation">,</span>b<span class="token operator">:</span><span class="token string">'2'</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

                
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